1. Field of the Invention
The present invention relates to a radio bus system for signal transmission over airwaves, and more particularly, to a system using spread spectrum according to a PN series (pseudorandom-number series) in an enlarged radio bus system formed by combining a radio bus system with an existing electrical line bus system. A spread spectrum signal is superposed on an electrical line bus by using an identical signal format for the spread spectrum modulated signal for each system. The term "radio" as here defined means broadly a transmission using an electromagnetic wave, including a light wave.
2. Description of the Prior Art
For controlling a great number of devices in remote locations it is not only uneconomical to connect a controller to each of the individual devices through wires, but it is also often impracticable to stretch such wires indoors. Therefore, a system for transmitting a control signal through existing electrical power lines as a home bus has been proposed in the prior art. For use in such a system, a spread spectrum (SS) communication method has already been proposed (Japanese Patent Application No. 017359/1984 and Japanese Patent Unexamined Publication No. 1613996/1985).
However, since the original object of a house's electrical line is to supply commercial electric power, there is no consideration in such systems for high-frequency transmission characteristics. Further, since various kinds of electric power appliances are connected to these lines, the characteristics of the lines vary according to the load, which may cause poor transmission. Also, noises produced from those appliances which are connected to the lines adversely affect signal transmission. Moreover, if the energy of a signal is increased in an effort to overcome noise, the increase of signal energy may adversely influence the various kinds of appliances which may be connected to the lines.
A bus system for an SS communication system of the prior art is generally shown in FIG. 1. A controller 5 and devices 6 and 7 and the like which communicate over the SS communication system are connected to an electrical line 1 through AC outlets 2(1), 2(2), 2(3) and so forth. The allocation of the controller and devices is arbitrary. For example, the number of controllers may be more than one. Also, the controller need not always be provided separately from the devices; it may be directly connected to the device. The communication equipment (inclusive of the controller and the devices) thus configured can transmit signals to and from one another through coupler circuits 3(1), 3(2), 3(3) and the like, and spread spectrum modems (hereinafter abbreviated as "SS modems") 4(1), 4(2), 4(3) and the like can be used in the SS communication system with electrical line 1 as a signal bus.
This bus system of the prior art is useful as a transmission system between different pieces of communication equipment connected to the electrical line. However, due to the nature of the system, it is necessary that each piece of equipment be fixed to the electrical line. Accordingly, such a transmission system is inconvenient. If such equipment could be remotely controlled by a portable apparatus apart from the electrical line, or, in other words, if a radio bus system could be formed, the transmission system would be much more useful.